It is well known to investigate the properties of underground formations surrounding a borehole by making various physical measurements of the formation from within the borehole. Such measurements are particularly useful when drilling wells such as oil and gas wells for identifying formations containing fluids of interest (oil and gas). In one technique, one or more measurement tools (typically known as ‘sondes’) are suspended on a cable and pulled along the borehole by the cable while making measurements. This is known as wireline logging. More recently, measurement tools have been built into the drilling tools used to drill the borehole in the first place. In this way, measurements can be made during the drilling process, known as ‘logging while drilling’.
Traditionally, formation properties such as porosity and fluid saturation are estimated from borehole geophysical measurements through the solution of a series of response equations relating different measurements to these formation properties. In the most common approach, these response equations are solved sequentially. For example, a density measurement is used to estimate porosity and then, based on this porosity, a resistivity measurement is used to compute water and hydrocarbon saturations. However, in order to accurately compute porosity from a density measurement, it is necessary to know the fluid density. In the presence of hydrocarbons, this fluid density is a composite of the pore filling water and hydrocarbon densities based on their respective saturations. Therefore, in order to compute an accurate porosity, it is required that the fluid saturations are known. Iterative approaches have been employed to solve this dependent problem. Another approach that has been employed is to solve these response equations simultaneously. Although this approach does allow for more accurate computation of porosity and fluid saturations, it still necessitates an understanding of the properties of the individual formation components; in the example above these include the density of the water and the hydrocarbon, and the resistivity of the water.
Formation saturation evaluation by resistivity and/or non-resistivity based methods such as formation capture cross section have been in use for a number of years and are well documented in the literature.
Recent developments in the design of logging while drilling tools mean that it is now possible to make simultaneous measurements of various formation properties at substantially the same location in the well before fluid invasion from the borehole into the formation begins to affect these properties. One such tool is described in WELLER, Geoff, et al. A New Integrated LWD Platform Brings Next-Generation Formation Evaluation Services. SPWLA 46th Annual Logging Symposium, New Orleans, USA. 26-29 Jun. 2005, p. 1-15. The use of this tool to provide resistivity and formation capture (sigma) measurements to be made and used in combination to compute both formation water resistivity and formation water saturation.
It is an object of this invention to provide techniques using simultaneous, multiple measurements in an appropriate fashion to solve for various formation properties without the requirement for additional a priori information normally required.